The present invention is directed to integrated circuits. More particularly, the invention provides a device and method for comparing voltages. Merely by way of example, the invention has been applied to an wide input common mode voltage comparator. But it would be recognized that the invention has a much broader range of applicability.
Integrated circuits or “ICs” have evolved from a handful of interconnected devices fabricated on a single chip of silicon to millions of devices. Current ICs provide performance and complexity far beyond what was originally imagined. In order to achieve improvements in complexity and circuit density (i.e., the number of devices capable of being packed onto a given chip area), the size of the smallest device feature, also known as the device “geometry”, has become smaller with each generation of ICs. Semiconductor devices are now being fabricated with features less than a quarter of a micron across.
Increasing circuit density has not only improved the complexity and performance of ICs but has also provided lower cost parts to the consumer. An IC fabrication facility can cost hundreds of millions, or even billions, of dollars. Each fabrication facility will have a certain throughput of wafers, and each wafer will have a certain number of ICs on it. Therefore, by making the individual devices of an IC smaller, more devices may be fabricated on each wafer, thus increasing the output of the fabrication facility. Making devices smaller is very challenging, as a given process, device layout, and/or system design often work down to only a certain feature size.
An example of such a limit is the performance of a voltage comparator. For example, voltage comparators can be integrated within a variety of integrated circuits. Wide-input common mode voltage comparators often utilize two comparators, an NMOS comparator to accommodate high common mode voltages and a PMOS comparator to accommodate low common mode voltages. The outputs of the two comparators are combined together within a summing circuit to create a single-ended output. However a delay may occur between the outputs of the comparators as a result of the type of circuits used in the NMOS and PMOS comparators.
From the above, it is seen that an improved design for a wide-input common mode voltage comparator is desired.